1. Field of the Invention
The present invention relates to an apparatus for and a method of controlling a fixing device in an electrophotographic recording system.
2. Description of the Related Art
With a conventional control method and a control apparatus of the type described above, the temperature of a fixing device is first measured by a thermal sensor to monitor its value, and after a determination is made as to whether or not the temperature is at a favorable level, the fixing device is held in its state for a fixed period of time set by the timer. For example, in a case where the desired fixing temperature is set at 180.degree. C., when a value of 179.degree. C. is obtained by monitoring at a fixed period of time after turning on the fixing device, the fixing device is controlled to remain in its on state. After a fixed period of time set by the timer has elapsed, e.g. 0.3 second, the temperature of the fixing device is monitored again, and if the temperature is detected to have reached 181.degree. C., the fixing device is controlled to an off state. Then, if the value of the temperature monitored after another fixed period of time is 180.degree. C., the fixing device is held as it is in the off state. However, if it is 179.degree. C., the fixing device is controlled to the on state.
However, with such a control method wherein a determination is made by monitoring the temperature at fixed intervals by means of a timer whether the temperature is above or below a fixed level, there is a high frequency of changeover between the on and off states of the fixing device. As a result, a voltage drop is liable to occur due to an inrush of current to a load (e.g., a halogen lamp), possibly exerting an adverse effect on peripheral equipment. In addition, there are cases where the on/off operation of the fixing device is effected by making an erroneous determination due to very small noises or the like on the line of a thermal sensor for monitoring the temperature. An externally mounted circuit becomes necessary in order to prevent this from occurring. Furthermore, there are cases where, due to a slight delay in the monitoring timing, the fixing device fails to be controlled to the off state until its temperature reaches a higher temperature than is required, or fails to be controlled to the on state until it conversely reaches a lower temperature than is required.